With the rapid market expansion in the fields of laptop computers and cellular phones, there has been a growing demand for small-sized, high-energy-density and high-capacity secondary batteries for use in such machines or instruments. To meet such demand, secondary batteries which utilize an electrochemical reaction associated with a charge transfer, with alkali metal ions such as lithium ions serving as charge carriers, have been developed. In particular, lithium ion secondary batteries are utilized as high-capacity secondary batteries high in energy density and excellent in stability in various electronic machines and instruments. In such lithium ion secondary batteries, a lithium-containing transition metal oxide is generally used as an active material in the positive electrode, and carbon as an active material in the negative electrode, and charging and discharging are carried out by utilizing the insertion and elimination reactions of lithium ions into and from these active materials.
In recent years, secondary batteries in which radical compounds are utilized as electrode active materials directly contributing to an electrode reaction have been of great value, and an electric storage device containing a nitroxyl compound which takes a nitroxyl cation partial structure in an oxidation state and takes a nitroxyl radical partial structure in a reduction state in the positive electrode have been proposed for further capacity increases (see Patent Document 1).
Patent Document 1 discloses a method of producing the nitroxyl compound in which polymethacrylate having a predetermined cyclic imino group in a side chain is dissolved in dichloromethane and oxidized by using meta-chloroperbenzoic acid. However, meta-chloroperbenzoic acid is expensive and highly dangerous, and refinement of its product requires complicated processes.
As a method of producing a nitroxyl compound more inexpensively and more safely, there has been known a method in which a corresponding imino compound is oxidized by using hydrogen peroxide (see Non-Patent Document 1 and Patent Document 2).
Non-Patent Document 1 discloses, for example, a method in which a polymethacrylic acid imino compound is oxidized by a hydrogen peroxide solution in a methanol solvent, which is a good solvent of the polymethacrylic acid imino compound, in the presence of sodium tungstate, and then ether is added to deposit the polymethacrylic acid imino compound. Patent Document 2 discloses a method in which a polymer having a secondary amine structure in a side chain is dissolved in an organic solvent having a low solubility in water and forming a 2-phase system with water, and then the polymer is oxidized by using hydrogen peroxide in the presence of water-soluble oxide catalyst.
However, these producing methods have various problems. In the producing method according to Non-Patent Document 1, for example, there are some problems such that a long time period is needed for the reaction, and that a catalyst is mixed into a polymer in the following refining step. Also, in the producing method according to Patent Document 2, since there is not only a problem that combined use with organic solvents, which are good solvents, needs a separation step, but also a problem that an insufficient reaction rate results in failure to obtain a sufficient radical conversion ratio. Moreover, a safer producing method is desirable so as to improve working environment upon production and to prevent an environmental pollution caused by waste water.
In addition, due to the desire for improving in performance stability of the secondary batteries, an electrode active material which is inhibited from eluting into the solvent contained in an electrolyte has been desired to be proposed; however, in accordance with the conventional techniques, it has been difficult to efficiently produce the nitroxyl compound by using a crosslinked material which is expected to be superior in stability to solvents.    Patent Document 1: Japanese Kokai Publication 2002-304996    Patent Document 2: Japanese Kokai Publication 2005-97409    Non-Patent Document 1: J. Polym. Sci. Polym. Chem. Ed., 10, 3295 (1972)